1. Field of the Invention
The present invention relates to linear motors, especially those linear motors which have moving coils.
2. Description of the Prior Art
Linear drives are used in many areas, including automation and robotic positioning systems, printers and disk drive units. Early designs often used lead screws or rack and pinion drives to provide the linear movement. Lead screws are generally limited to low speeds and low accelerations, have backlash between the ball nut and the screw, require periodic maintenance and require larger screw diameters as the length of travel increases. Rack and pinion drives are often speed and acceleration limited and contain backlash. Where zero backlash was required in these sorts of units, elaborate anti-backlash techniques and apparatuses were developed to resolve the problem, but these techniques added further maintenance and adjustment problems.
To overcome some of these problems, linear motors were developed. One type of linear motor is the linear stepper motor, which is the equivalent of a rotary stepper motor. Linear stepper motors overcame the problems of low speeds and accelerations, but only when moving very small loads, because the static force developed by linear stepper motors is typically on the order of ten pounds or less. In certain applications, this static force output is insufficient to overcome the static and dynamic friction created by the load. In addition, an air gap between the forcer, corresponding to the rotor, and the platen, corresponding to the stator, needs to be rigidly maintained with small variations, requiring strict manufacturing tolerances of the bearing or support system and resulting in high maintenance needs.
A second type of linear motor is the moving magnet motor. These motors incorporate a series of stacked ferromagnetic laminations with the wire forming the coils wound integrally into the laminations. These motors are the linear equivalent of standard rotary, brush type direct current motors. The linear slider, corresponding to the rotor, incorporates several permanent magnets, and is held a fixed air gap away from the laminations, corresponding to the stator. This air gap is generally larger than that of the linear stepper motors, reducing to some extent the manufacturing tolerances and bearing or support system maintenance requirements. These motors are abe to produce very large forces, up to 1,000 pounds, but have several problems. The force from the stacked lamination motors varies as they travel, due to a ripple effect or interaction between the wire, the laminations, and the permanent magnets and their various alignments. The magnets are often skewed or angled with respect to the laminations to help reduce this ripple force, but this angling does not eliminate the problem. Additionally, there is a very large attractive force between the slider and the laminations, often two to two and a half times as great as the linear or drive force being generated. This large attractive load between the slider and the laminations results in a much more complicated bearing system because of these forces and the need to maintain the air gap against these large attractive forces. Additionally, the motor length is generally limited to less than three feet because of problems maintaining the laminations at the required flatness, with joining of additional segments not easily performed.
Yet another type of linear motor is the moving coil linear motor. These can be either brushed or brushless designs and have a moving coil passing through an air gap created by either two rows of permanent magnets and magnetic circuit completion means or back iron or one row of permanent magnets and a magnetic circuit completion means using one back iron and one ferromagnetic bar. While there are large attractive forces between the two rows of magnets or the row of magnets and the magnetic circuit completion means, the air gap is maintained by the use of steel or aluminum bars to support the magnets and the magnetic circuit completion means. In addition, the force on the moving coil assembly is generally very low in directions other than the intended direction of movement because in many cases there is no ferromagnetic material located in the coil. In most cases, the coil assembly contains aluminum support materials which are not attracted by the magnets, but are subjected to induced eddy currents during the motion of the moving coil. This eddy current development acts as a small negative force proportional to the speed of travel and so reduces the efficiency of the motor.
However, the moving coil windings in these motors are generally the limiting factor to the force that can be developed because of heat buildup in the windings. The linear force developed is proportional to the current passing through the windings, the number of turns of wire and the flux density of the magnetic circuit. Given a constant flux density and a given number of windings, force is then directly proportional to the current in the windings. At the same time, power used or heat needed to be dissipated is proportional to the current squared and therefore the heat developed builds up at a rate much greater than the increase in force. This generally results in a current limitation in the coils being required to prevent overheating of the coil assembly.
The prior art moving coil linear motor designs were not conducive to heat removal because the coil assemblies were generally only air-cooled and had poor heat sinking of the coil assemblies. Additionally, the permanent magnets used in the motors were often quite expensive because of the combination of the high flux densities desired and the number of magnets required per given length to develop those densities.
It is desirable to have a linear motor which develops large accelerations, static force and speeds and yet does not have any ripple effects, does not require large numbers of expensive magnets and does not have coil assemblies which easily overheat.
U.S. Pat. No. 4,318,038 a moving coil linear motor which has two rows of alternating magnets with a moving coil assembly located on a central ferromagnetic bar. The central ferromagnetic bar is located between the two rows of magnets to form two sets of magnetic circuits. The use of two different coils or poles is stated to result in a reduced danger of the coils being burnt and overheated in use.
U.S. Pat. No. 4,151,447 discloses a linear motor utilizing one or two rows of alternating, permanent magnets affixed to a ferromagnetic, U-shaped bar which supports the magnets and provides the necessary ferromagnetic material for magnetic circuit completion. A series of coils is located between the magnets and energized to cause the coils to move. The coil faces are preferably parallel to the direction of movement of the coil assembly.
U.S. Pat. No. 4,641,065 discloses a moving coil linear motor having a single row of alternating, permanent magnets with a backing iron and an opposing ferromagnetic bar to form a closed magnetic circuit. A coil couple of given dimensions relative to the magnets is used to produce the linear force. The coils form a U shape around the magnets to reduce the total magnetic circuit air gap. The motor commutation is provided by various arrangements of brushes, contacts or optical sensors to simply commutate the coils or to allow development of an alternating current output providing position and speed feedback.
U.S. Pat. No. 4,460,855 discloses a multi-pole, multi-phase, moving coil linear motor. The coil assembly is formed on a cylindrical object located around a cylindrical magnet series. The magnets are arranged in an alternating pole sequence with gaps of approximately the magnet length between adjacent magnets. It is specifically indicated that there are no laminations in the coil assembly, resulting in a lightweight armature. Position feedback can be developed by use of a light source, a photocell and a graticle and appropriate electronic circuitry.
U.S. Pat. No. 4,220,899 discloses a cylindrical linear motor. A central, laminated, ferromagnetic cylinder used for magnetic circuit completion has multi-pole, multi-phase coils wound around it. The structure is encircled by a series of permanent magnets. The magnets are closely encircled by an equivalent to the standard back iron. Various commutation and drive methods are disclosed.
U.S. Pat. No. 4,408,138 discloses a linear stepper motor having one set of fixed magnets and a series of stepper windings positioned in a lamination body having varying sizes of teeth.
U.S. Pat. No. 4,560,911 discloses a linear motor for use with a positioning table. It is disclosed that the motor uses one set of permanent magnets and one set of coil pairs, either of which can be moving while the other is held in fixed location. The fixed assembly can have a series of sets or poles. The motor uses brushes to change the voltage being applied to the coils to provide for movement of the motor. The coils are located in a toothed arrangement with laminations.